`sqrt{11+6sqrt2} - sqrt{11-6sqrt2}`
`= sqrt{sqrt9^2+2*3sqrt2+sqrt2^2} - sqrt{sqrt9^2-3*2sqrt2+sqrt2^2}`
`= sqrt{(3+sqrt2)^2} - sqrt{(3-sqrt2)^2}`
`= |3+sqrt2| - |3-sqrt2|`
`= 3 + sqrt2 - (3-sqrt2)`
`= 3 + sqrt2 - 3 + sqrt2`
`= 2sqrt2`
Hằng đằng thức :
`a^2 + 2ab + b^2 = (a+b)^2`
`a^2 - 2ab + b^2 = (a-b)^2`